Telephone and pipeline companies, electric, water and gas utilities and others that have underground lines or subterranean installations have need for locating and identifying underground lines and installations to service and maintain them. Such companies also have to guard against accidental damage to the underground lines and installations when it is necessary to carry out repairs or when any excavation is undertaken by others who are not aware of the underground lines and installations.
Marking tapes such as those described in U.S. Pat. Nos. 3,115,861; 3,282,057; 3,504,503 and 3,633,533 have been used. Such tapes are colored, printed and are made of various plastic materials. The tapes are designed so they will tear when pulled up, for example, by a mechanical digger used in an excavation operation. When so constructed, the presence of such tapes beneath the surface of the earth cannot be detected by any detecting instruments and are revealed only in connection with a digging operation. U.S. Pat. No. 3,633,533 addressed this detection problem by adding a flexible, metal film to the tape structure wherein the metal film is protected from moisture, oxidation or other deterioration when buried in soil. It has been found that such tapes must be buried within about one foot of the earth's surface and, except for being exposed when dug up, can be detected only by a metal detector. While it is indicated such tapes are traceable with standard radio frequency (RF) path tracing equipment, the tapes can in fact only be traced for short distances depending upon the frequencies used. The distances may be a few hundred feet or less than one hundred feet. Further, detection of the location of a break in the tape at a point remote from the break is not possible with the marking tape structure of U.S. Pat. No. 3,633,533.
A single tracing wire is sometimes buried with a utility line wherein ground is used as a return with an alternating current signal applied between the wire and ground when the wire is to be traced. An electromagnetic or "H" field is produced along the wire by the current flow in the wire. This "H" field is detected by a receiver carried by an operator above the wire to trace the wire. The use of a common mode current, i.e., a common return path for the current, in this case via ground, results in ambiguities caused by adjacent structure and phantom cables. Such an arrangement does not provide the necessary structure for detection of a break in the wire from a point remote from the break and does not provide an early visible warning to someone digging where the utility line is buried as in the case of the marking tapes.
U.S. Pat. No. 4,119,908 discloses a method for tracing utility lines and locating other buried objects wherein an underground conductor is provided in the vicinity of the buried utility line and passive markers are disposed adjacent and alongside the conductor. An alternating current signal is applied to the conductor allowing the conductor to be traced as indicated above in the case of a tracing wire. A receiver when swung to and fro laterally of the energized conductor will normally produce an output having a peak-null-peak sequence. The passive marker has a tuned circuit that is tuned to the alternating current signal so that the output of the receiver presents a peak when the receiver reaches a point where a null should be produced to provide a precise location of the passive marker. This method has the same shortcomings as those noted for the tracing wire method.